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Have you checked the water levels in the batteries? There are two rectangular caps between the posts on the battery. Pop them off carefully, and you’ll see three holes that have water in them. Try to use only distilled, but hard times is hard times. The battery won’t charge if there’s not enough water to conduct the current.

Thank you, my partner probably checked but I'll make sure. And luckily I have distilled water

Just checked, it has water.

Dead cell more than likely or your alternator is toast.

I'm hoping it's just the battery, but I realize it might be the alternator. I'm just wondering how batteries work, like if they're needed when the vehicle is running. If we get it started but it won't charge can I still drive?

Your alternator may not produce enough voltage to power your electrical system fully. In older cars this was ok because you didn’t have such a high demand for power unlike todays cars with power windows, steering, heated seats, cameras, etc. best err in the side of caution, and have the battery and the alternator tested at a parts store.

Thank you! I'll do that